Proud Prince William immediately called the Queen, his father Prince Charles, and his brother Harry and made sure Kate’s family was told the happy news.
After more than a week of waiting, Kate was admitted to the Lindo Wing of St Mary’s, Paddington, west London, at 5.30am, already in labour.
Just under 11 hours later the baby, who is third in line to the throne after Charles and William, arrived at 4.24pm.
Palace sources said the couple chose to delay the public announcement of the birth until 8.30pm to allow them to spend “quality time” together.
Outside Buckingham Palace, thousands of well-wishers from around the globe who had gathered through the day broke into cheers of “hip-hip hooray!”
Kate, 31, and William had shunned a police escort as they were driven to the exclusive west London maternity unit.
A team of royal protection officers rushed the mum-to-be into the hospital via a rear exit, the same route taken by Princess Diana when she gave birth to William on the same ward in 1982.
Someone who saw the pair enter said: “Kate looked quite calm. They weren’t on view more than a minute.Nobody would have known it was them.”
William, 31, later said in a statement: “We couldn’t be happier.”
The Queen returned to London from Windsor Castle after asking officials to keep her informed.
Prince Charles, 64, said: “Both my wife and I are overjoyed at the arrival of my first grandchild.
“It is an incredibly special moment for William and Catherine and we are so thrilled for them on the birth of their baby boy.
“Grandparenthood is a unique moment in anyone’s life, as countless kind people have told me in recent months, so I am enormously proud and happy to be a grandfather for the first time and we are eagerly looking forward to seeing the baby.”